1. Technical Field
This application is the US national phase of international application PCT/GB2005/000633 filed 21 Feb. 2005 which designated the U.S. and claims benefit of GB 0404196.8 and GB 0406513.2, dated 25 Feb. 2004 and 23 Mar. 2004, respectively, the entire content of which is hereby incorporated by reference.
The present invention relates to overload control in a communications network, particularly but not exclusively to an external overload control system for a Media Gateway Controller (MGC) to restrict the rate at which offered calls are received by the MGC from Media Gateways (MGs) in a VoIP network.
2. Related Art
Tele-voting (in which a telephone number is broadcast for users to call to register their vote) and similar mass calling schemes often result in very high call rates which have a sudden onset and which last for a relatively short duration. It is not economic to provision sufficient network capacity to cope with such overwhelming surges in traffic, necessitating overload control to be implemented within the communications network to enable emergency and other core services to be supported. However, traditional methods to cope with sudden surges in the number of calls sent to a specific network address are not satisfactory as the communications networks themselves evolve.
Conventional methods of providing overload control in a communications network include call gapping techniques which seek to limit the number of calls made. Such call gapping techniques are well known to those skilled in the art and involve barring or blocking calls received within a predetermined interval of time (the gap) following a first call which triggers the onset of the gap. An example of a call-gapping technique is described for example in U.S. Pat. No. 6,259,776 entitled “System for controlling telecommunications overload traffic”, the contents of which are hereby deemed incorporated into the description by reference.
However, traditional telecommunications networks are evolving to offer more functionality and to support differing media from that offered by conventional public switched telephone networks (PSTNs). For example, call concentrators in PSTNs can be replaced by access media gateways (MGs) which convert conventional copper line to provide access to Internet Protocol (IP) media transport. In such communications networks, the MGs are controlled by Media Gateway Controllers (MGCs) which perform a traffic analysis role by analysing dialled digits to determine the routing of calls, analogous to the local exchange processors implemented in conventional PSTNs. More details on MGs and MGCs can be found from the Media Gateway Control (MEGACO) Charter standard documentation available from the Internet Engineering Task Force (IETF) standardisation body (url http://www.ietf.org/html.charters/megaco-charter.html).
Whenever a destination telephone number is advertised on a national basis and a significant number of customers attempt to make a call to the telephone number, a focused overload of calls seeking to use incoming trunks to the destination main switching unit and/or the destination local exchange can result in switch blocking of normal service traffic. Several techniques have been proposed to deal with such problems within conventional PSTNs, such as U.S. Pat. No. 6,259,776 proposes for example. U.S. Pat. No. 6,259,776 describes a telecommunications network including an overload control arrangement in which the overload control arrangement restricts call connections to a predetermined destination when traffic to such a destination exceeds a predetermined level. The arrangement comprises a plurality of identical overload control functions each running in a respective one of a plurality of nodes of the network and each having a respective gapping period determined from the perceived overload level at the respective node, the overload control functions exchanging data defining their respective gapping periods and effecting adjustment towards an average gapping period so that substantial differences between respective gapping periods from respective nodes to any one predetermined destination are avoided.
Whilst the overload control system described in U.S. Pat. No. 6,259,776 provides an effective solution in a conventional telecommunications SS7-type network, however, it is less effective for communications over an Internet Protocol (IP) network or similar type of network where a large number of network access points A1 . . . AN) may be under the control of a single network access controller X1, such as FIG. 1 of the accompanying drawings shows. In such situations, the critical overload condition for the network is related to the maximum call processing capacity of the controller X1, which has only a finite amount of resources available to process calls seeking admission to the network. This limitation on the number of calls admitted to the network is shown schematically in FIG. 2 of the accompanying drawings.
In the graph shown in FIG. 2 the x-axis represents the rate of calls offered to the network access controller by the network access points and the y-axis represents the number of calls admitted by the network access controller to the network. The total number of aggregated calls offered by all of the access points A1 . . . AN which are actually admitted by the controller X1 to the communications network as a function of the aggregated offered rate is shown by the solid line plot (thus this shows the rate of calls admitted to the network). Where the offered rate is relatively low, the admission rate is able to rise to match the number of calls offered. However, the controller has only finite resources and as the use of its resources increases, eventually the controller becomes overloaded. This occurs at the point marked A in FIG. 2, and at this point, the controller needs to reject a certain proportion of off-hook signals received to enable response times to remain relatively low.
As the number of new calls offered per second, i.e. as the offered call rate, increases beyond point A, the admission rate fails to rise as sharply, and finally the admission rate for calls to the network reaches a maximum for a given rate of offered calls LM. Beyond this point, the resources of the controller become increasingly involved with rejecting offered calls as opposed to admitting calls. Eventually, when the number of offered calls reaches rate LC all of the controller's resources will be occupied in rejecting calls, and no new calls will be admitted.
The network access controller's internal control mechanisms are reflected in the diagram shown in FIG. 2. The access controller's internal control provides the ability to reject some or all of the offered load, and provides no ability to regulate any external restriction (such as gapping) on the offered load.
The dashed curve in FIG. 2 shows the response time of the network access controller to the signals (for example, off-hook signals) that it receives from the network access points within its domain of control. Initially, prior to the overload point being reached, the controller will have a slowly rising response time as it steadily processes more and more offered calls. The ability of the access controller to reject offered calls needs to be coupled with an effective external restriction if the access controller is to regulate its response times. Whenever the traffic offered to the network access points exceeds LM it is necessary to implement some form of adaptive external restriction control to ensure the rate offered to the access controller is held relatively close to LM to maximise the access controller's throughout.
One form of adaptive external control known to those skilled in the art is that provided by a call gapping overload system. The call gapping process enables the load offered by the network access points to the access controller to remain around LM which enables the access controller's response time to remain relatively constant. However, if no external control is implemented, or if the external control is not sufficient to limit the offered call rate to the vicinity of LM, then if the offered rate rises until it approaches LC, the internal overload control process implemented by the overload controller will effectively reduce the access controller's throughput to zero, which would result in none of the offered traffic being admitted to the network.
Conventional call gapping processes also have other limitations. For example, if the process is applied within a system in which a large number of network access control points are controlled by a single access controller (also known to those in the art as a very high “fan-in”), the rate at which traffic is admitted by the external restriction (i.e. applied by the network access points) responds too slowly to commands from the control point to change the admitted rate (i.e. to change the gap interval if a gapping process is used to implement the external restriction). This slow response by the network access points results in the overload control servo loop being slow and possibly unstable.
Other sources of delay contributing to this slow response problem include:                delay in sending control messages out from the access controller to the network access points due to the large number of control messages which need to be sent;        the first offered call always being admitted by the external restriction in a conventional call gapping process when a call restriction is being initially imposed, which then generates a synchronisation effect if all of the network access points then have concurrently running active interval timers imposing a call gap; and        if a gap interval update is applied to an access point which is already being gapped, the delay waiting for an existing gap interval timer to expire before the updated gap can have an effect on the admitted rate.        
Thus the techniques imposed by conventional call gapping are no longer effective in situations where the critical overload condition occurs at an access controller which controls a number of access points, and more preferably a very large number, for example, several thousand.